1. Field of the Invention
The invention relates to an electric auxiliary heating for a motor vehicle. In particular, the invention relates to an auxiliary heating comprising, in addition to a heating element, also a control unit for adjusting the heating power that has to be generated by the heating element.
2. Description of the Related Art
For enhancing the comfort in a motor vehicle, heating systems have already been used for a fairly long time. In these heating systems, the air sucked in and discharged by a blower is heated making use of the waste heat of the engine or of additional heating modules and then blown into the vehicle passenger compartment. The heating power of such heating systems installed in fittings, seats or other parts of the vehicle is normally manually determined by the user, but the user does not have much influence on the precise temperature of the air discharged.
Since it is normally only possible to predetermine the heating power or a heating power stage, it is not always easy to achieve a desired room temperature in the passenger compartment. In the initial heating phase, very little heating power is often available e.g. due to a cold engine. Hence, the user first selects a very high heating power or he makes use of electric auxiliary heating systems. When the temperature of the engine increases, the air blown into the passenger compartment will, however, have a temperature that is much higher than the temperature desired by the vehicle passenger so that the latter will reduce the heating power of the auxiliary heating or the selected heating stage. In most cases, the heating power is, however, reduced to such an extent that the temperature in the passenger compartment will become too low, whereupon the heating power must be increased once more.
As can be seen from the above example, it may in some cases be very time consuming to accomplish an aimed-at temperature in the passenger compartment. This is made worse by the fact that the heated air is blown into the passenger compartment by a blower. In addition to the selected heating power, also the strength of the blower influences the temperature of the air flowing into the passenger compartment. Since users often vary the strength of the blower en route, further temperature fluctuations will be caused by the resultant change in the amount of air flowing through the heating. This makes it even more difficult to achieve a target temperature.
DE 103 17 512 discloses an air supply means for a motor vehicle in the case of which a temperature sensor is provided between a heating element arranged in the air duct and the air discharge opening. Taking into account the temperature measured by the temperature sensor, the heating power is varied in a simple way: if the measured temperature is too high, the heating power is reduced, if it is too low, the heating power is increased. However, the heating power control described in DE 103 17 512 leads to temperature fluctuations in the discharged air, in particular to overshoots beyond the aimed-at target temperature of the discharged air, which are unpleasant for the vehicle passengers.
Another drawback of the air supply means described in DE 103 17 512 is that a separate temperature sensor is required for measuring the temperature of the discharged air. Such a temperature sensor entails additional costs in the production process and increases the fault proneness.